This invention relates to a solid-state laser device and, more particularly, to a solid-state blue laser device comprising a semiconductor laser device as an excitation light source.
Solid-state blue laser devices are generally adapted to many applications such as a source for a high density optical disk, a measurement field for analyses, medical care and in so on, and an information processing field.
Various solid-state blue laser devices are already known. By way of example, a solid-state blue laser device is described by P. Gunter in a paper submitted to "1980 European Conference on Optical Systems and Applications (Utrecht)" in SPIE Vol. 236, pages 8-18, under the title of "Nonlinear optical crystal for optical frequency doubling with laser diodes." The solid-state blue laser device of Gunter comprises a semiconductor laser device formed by a semiconductor laser medium which has a composition represented by a chemical formula of Ga.sub.1-x Al.sub.x As. Such a semiconductor laser device will be called a Ga.sub.1-x Al.sub.x As laser device for the purpose of simplification. When x is equal to 0.05, the Ga.sub.1-x Al.sub.x s laser device emits, as an excitation laser beam, a laser beam having an emission wavelength of 860 nm at room temperature of 300 K.
The excitation laser beam is condensed or collected, as a condensed laser beam, onto a nonlinear optical crystal by a condensing optical system. The nonlinear optical crystal has a composition represented by a chemical formula of KNbO.sub.3. Such a nonlinear optical crystal will be referred to as a KNbO.sub.3 crystal for the purpose of simplification. The KNbO3 crystal converts the excitation laser beam having the emission wavelength of 860 nm into a converted laser beam having a converted wavelength of 430 nm. Therefore, the solid-state blue laser device of Gunter produces the converted laser beam having the converted wavelength of 430 nm as a blue laser beam. The solid-state blue laser device of Gunter is small in size and has light weight.
However, the solid-state blue laser device of Gunter is disadvantageous in that it produces the blue laser beam with low power. This is because the blue laser beam has power dependence upon that of the excitation laser beam and therefore lower than that of the excitation laser beam. In addition, the solid-state blue laser device of Gunter is defective in that it can not be operable in a Q-switching mode. This is because the solid-state blue laser device of Gunter has not a gain medium such as a laser medium which accumulates light energy.
Another solid-state blue laser device is described in an article which is published by G. J. Dixon et al on OPTICS LETTERS, Vol. 13, No. 2, pages 137-139, Feb. 1988, and which is entitled "Efficient blue emission from an intracavity-doubled 946-nm Nd:YAG laser." The solid-state blue laser device of Dixon et al comprises, as a solid-state laser medium, a 2-mm-long Nd:YAG laser rod which operates at 946 nm. The Nd:YAG laser rod is polished with a 2-cm convex radius on its incoming end surface and a flat on its outgoing end surface. The Nd:YAG laser rod is pumped or excited by a 588-nm cw ring dye laser device which emits, as an excitation laser beam, a laser beam having an emission wavelength of 588 nm at room temperature. The excitation laser beam is focused through the curved incoming end surface of the Nd:YAG laser rod with a 75-mm focal-length lens. Excited by the excitation laser beam, the Nd:YAG laser rod generates an excited laser beam having an excited wavelength of 946 nm. On the curved incoming end surface, a dichroic multilayer dielectric reflector is coated directly. The reflector has an antireflectivity greater than 99.8% at 946 nm and transmission in excess of 95% at 588 nm.
The solid-state blue laser device of Dixon et al further comprises, as a nonlinear optical crystal, a 2.5-mm KNbO.sub.3 crystal. The KNbO.sub.3 crystal is disposed between the Nd:YAG laser rod and a 2.5-cm-radius output coupler or mirror. Both end surfaces of the KNbO.sub.3 crystal and the flat outgoing end surface of the Nd:YAG laser rod are antireflection coated for 946 nm. The output mirror is coated with a multilayer dielectric stack having greater than 99.8% reflectivity at 946 nm, greater than 80% transmission at 1.06 .mu.m, and greater than 60% transmission at 473 nm. A combination of reflector and the output mirror is operable as a resonator in response to the excited laser beam. With this structure, the solid-state blue laser device of Dixon et al produces, as a blue laser beam, a beam having a wavelength of 473 nm. The solid-state blue laser device of Dixon et al can produce the blue laser beam having high power and is operable in a Q-switching mode. This is because the solid-state blue laser device of Dixon et al comprises the solid-laser medium which accumulates light energy.
However, the solid-state blue laser device of Dixon et al is disadvantageous in that it is bulky in size, has heavy weight, and has less reliability. This is because the solid-state blue laser device of Dixon et al comprises two optical components, namely, the solid-state laser medium and the nonlinear optical crystal. In addition, inasmuch as the nonlinear optical crystal comprises the KNbO.sub.3 crystal, the nonlinear optical crystal must be accurately made thermal insulation within allowable temperature range of 0.2 .degree. C.
A solid-state green laser device has been proposed in an article which is contributed by Lu Baosheng et al to Chinese Physics Letters Vol. 3, No. 9, pages 413-416 (1986) and which is entitled "Excited Emission and Self-Frequency-Doubling Effect of Nd.sub.x Y.sub.1-x Al.sub.3 (BO.sub.3).sub.4 Crystal." The solid-state green laser device of Lu Baosheng et al comprises, as a self-frequency doubling crystal, a crystal having a composition represented by a chemical formula of Nd.sub.x Y.sub.1-x Al.sub.3 (BO.sub.3).sub.4 and which thus includes Nd. Such a self-frequency doubling crystal will be called an Nd.sub.x Y.sub.1-x BO.sub.3).sub.4 crystal for the purpose of simplification. Inasmuch as Nd is operable as a laser activator, the Nd.sub.x Y.sub.1-x Al.sub.3 (BO.sub.3).sub.4 crystal is capable of emitting a primary laser beam which has a fundamental wavelength determined by Nd when the Nd.sub.x Y.sub.1-x Al.sub.3 (BO.sub.3).sub.4 crystal is excited or pumped by an excitation laser beam. The excitation laser beam may be referred to as a pumping laser beam. The primary laser beam will be called a fundamental laser beam. In addition, the article reports that wavelength conversion takes place within the Nd.sub.x Y.sub.1-x Al.sub.3 (BO.sub.3).sub.4 crystal so as to partially convert the primary laser beam of the fundamental wavelength into a subsidiary laser beam of a harmonic wavelength which is derived from the Nd.sub.x Y.sub.1-x Al.sub.3 (BO.sub.3).sub.4 crystal and that the Nd.sub.x Y.sub.1-x Al.sub.3 (BO.sub.3).sub.4 is capable of emitting the subsidiary laser beam of the harmonic wavelength. The subsidiary laser beam may be referred to as a harmonic laser beam. Practically, when x is smaller than 0.2, the fundamental wavelength of 1.064 .mu.m is stably converted into a second harmonic wavelength of 0.532 .mu.m.
In order to carry out such wavelength conversion, the self-frequency doubling crystal is combined in the solid-state green laser device of Lu Baosheng et al with a resonator and an exciter for supplying an excitation laser beam to the self-frequency doubling crystal. In the solid-state green laser device of Lu Baosheng et al, the resonator comprises an output mirror and a reflection mirror so as to provide an outer resonator for the primary laser beam of the fundamental wavelength. Both of the output mirror and the reflection mirror are opposed to both ends of the self-frequency doubling crystal with spacings left therebetween, respectively. In addition, the exciter is formed by a dye laser device directed to a side surface of the self-frequency doubling crystal with a distance left between the side surface and the dye laser.
In the solid-state green laser device of Lu Baosheng et al, each of the output mirror and the reflection mirror has an optical characteristic such that about 100% of the primary laser beam is reflected. In addition, the reflection mirror reflects about 100% of the subsidiary laser beam while the output mirror transmits 80% of the subsidiary laser beam.
Under the circumstances, when the excitation laser beam is supplied from the dye laser device to side surface of the self-frequency doubling crystal, a resonance path for the primary laser beam of the fundamental wavelength is formed between the output mirror and the reflection mirror. Subsequently, the primary laser beam is partially converted in wavelength into the subsidiary laser beam within the resonance path. As a result, the subsidiary laser beam is transmitted through the output mirror as the green laser beam.
However, the solid-state green laser device of Lu Baosheng et al is disadvantageous in that it is bulky in size and has a short life time. This is because the dye laser device is used as the exciter. Furthermore, since a dye should be exchanged from time to time, a lot of labor is necessary for maintenance of the solid-state green laser device.
In addition, the resonator is formed by the output mirror and the reflection mirror both of which are distant from the self-frequency doubling crystal. Such a resonator is large in size and therefore results in an increase of a size in the solid-state blue laser device. Moreover, the primary laser beam is objectionably weak in strength. This is because a loss inevitably takes place while the primary laser beam is propagated within a space between the output mirror and the reflection mirror. This results in a reduction of a conversion efficiency between the primary laser beam and the subsidiary laser beam.